Multiple Input Multiple Output (MIMO) is a transmission method involving multiple transmit antennas and multiple receive antennas which promises to greatly increase the link capacity of wireless communication systems. Various transmission strategies require the transmit array to have some level of knowledge concerning the channel response between each transmit antenna element and each receive antenna element, and are often referred to as “closed-loop” MIMO. Obtaining full broadband channel knowledge at the transmitter is possible using techniques such as uplink sounding in Time Division Duplexing (TDD) if array calibration is performed at the transmitter. However the uplink sounding method does not work in Frequency Division Duplexing (FDD) systems, when a relay is used in a TDD system, or when array calibration is not performed at the transmitter. Thus there is a need for method of efficiently providing full channel knowledge at the transmitter when uplink sounding is not effective for that purpose.